This invention relates in general to continuous casting of thin metal strip by a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle or series of delivery nozzles (also called the “core nozzles”) located above the nip, forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls so as to dam the two ends of the casting pool against outflow.
Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into a smaller vessel before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip.
During operation, the metal flow rate and molten metal temperature in the area where the side dams, casting rolls and meniscus of the casting pool intersect, i.e. the “triple point” area or region, is controlled. Notably, the distance between the side dams and the ends of the delivery nozzles nearest the side dams should be controlled and maintained to prevent the formation of unwanted steels skulls either on the side dam or delivery nozzle.
Apparatus and method for controlling and maintaining a set distance between the outer ends of the delivery nozzles and the side dams during a campaign is disclosed in U.S. Pat. Nos. 6,910,523, 6,588,492, 7,147,035. The apparatus and method disclosed has a carriage assembly for commonly supporting the side dams and nearest delivery nozzles to maintain distance between the side dams and ends of the delivery nozzles at a set distance with wear of the side dams. The delivery nozzles could be moved relative to the side dams by the carriage assembly. The movement also involved simultaneously moving of both delivery nozzle and the adjacent side dam to maintain the distance between the side dam and end of the delivery nozzle. This movement affects the side dam force and thus side dam wear. Further, the movement of the side dam by the support to compensate for wear of the side dam required repositioning of the delivery nozzle to maintain the distance between the side dam and the end of the nearest delivery nozzle.
Solidified skulls may form from time to time on the side dam and also the delivery nozzle when the distance between the side dam and nozzle is not maintained. Additionally, skull formation is affected by flow patterns within the casting pool and temperature variations in the casting rolls and side dams. When these skulls drop into the roll nip, they cause the two solidifying shells at the casting roll nip to separate and “swallow” additional liquid steel between the shells causing the strip surface to reheat and causing the strip to break thus disrupting the continuous production of coiled strip. The dropped skulls at the nip are known as “snake eggs” and are detected as horizontal force spikes at the roll nip as well as visible bright bands across the width of the strip. Snake eggs also apply resistive forces against the side dam in addition to the forces generated by the ferrostatic head in the cast pool and can thus cause the side dam to lift from the casting roll edge resulting in the leakage of steel between the side dam and the casting roll necessitating termination of the casting sequence. Additionally, snake eggs passing through the nip between the casting rolls can cause lateral movement of the casting rolls and also cause movement in the side dams. To resist the increased forces generated by the snake eggs and the stiction of the side dam apply cylinders, the side dams are typically applied to the casting rolls with higher forces, thus increasing side dam wear.
We have found that improved flow within the molten pool and a reduction of skulls can be achieved by utilizing side dams with an improved shape during a casting campaign. The improved side dams have been found to allow for improved flow patterns within the casting pool and improved temperature control of the side dams. Improved flow patterns and temperature control, especially in the triple point pouring region, has led to a reduction in the occurrence of skulls and the incidence of snake eggs.
Disclosed is an apparatus for continuously casting metal strip which includes a pair of counter-rotatable casting rolls laterally positioned to form a nip there between through which thin strip can be cast, a pair of confining side dams adjacent the ends of the casting rolls capable of confining a casting pool of molten metal supported on the casting rolls and formed on the casting surfaces above the nip, each side dam has a surface capable of contacting the molten metal of the casting pool, the surface including an unraised portion and a raised portion with the unraised portion forming a base between the raised portion of the side dam and the casting surfaces of the casting rolls to guide the flow of molten metal, a metal delivery system disposed above the nip and capable of discharging molten metal to form the casting pool supported on the casting rolls.
The raised portion may form a trough of substantially constant width with the unraised portion as base between the raised portion and the casting rolls or a trough of substantially varying width with the unraised portion as base between the raised portion and the casting rolls. In the case of a varying width trough, the trough width may be greater toward the meniscus and less toward the nip. The trough width may vary by at least about 5 mm or by at least no more than about 25 mm. In any case, the width of the trough may be between about 5 mm and about 25 mm. The raised portion may extend in height from the unraised portion forming a trough at least about 3 mm or about 15 mm or less in depth. The troughs may extend in length from the tops of the side dams and may extend to the nip.
Also disclosed is a method of continuously casting metal strip which includes the steps of assembling a pair of counter-rotatable casting rolls to form a nip there between through which thin strip can be cast, assembling a pair of confining side dams adjacent the ends of the casting rolls capable of confining a casting pool of molten metal supported on the casting rolls and formed on the casting surfaces above the nip, each side dam has a surface capable of contacting the molten metal of the casting pool, the surface including an unraised portion and a raised portion with the unraised portion forming a base between the raised portion of the side dam and the casting surfaces of the casting rolls to guide the flow of molten metal, assembling a metal delivery system disposed above the nip and capable of discharging molten metal to form the casting pool supported on the casting rolls and counter rotating the casting rolls so as to cause the formed troughs to guide the flow of molten metal adjacent the casting surfaces of the casting rolls to form solidified shells of the casting surfaces and form cast strip discharging downwardly from the nip.
The raised portion may form a trough of substantially constant width with the unraised portion as base between the raised portion and the casting rolls or a trough of substantially varying width with the unraised portion as base between the raised portion and the casting rolls. In the case of a varying width trough the trough width may be greater toward the meniscus and less toward the nip. The trough width may vary by at least about 5 mm or by at least no more than about 25 mm. In any case, the width of the trough may be between about 5 mm and about 25 mm. The raised portion may extend in height from the unraised portion forming a trough at least about 3 mm or about 15 mm or less in depth. The troughs may extend in length from the tops of the side dams and may extend to the nip.
Various aspects of this invention will become apparent from the following detailed description and accompanying drawings.